While D&D 4th edition had many good design goals, I feel as though the authors shot themselves in the foot either by not implementing mechanics that backed up those design goals, or implementing features that went completely contrary to their design goals. Here are some examples of those mechanics that were implemented badly, and cause 4e to be a poor game overall.

*Abilities feel very standardized from what I've played (I've only been in 4-5 games thus far, though one or two only lasted a few sessions), so people feel very similar.

*Battles feel like they last too long. Battles in my opinion should last 4-8 rounds, it feels like they last 10-20 rounds in 4e which can easily lead to boredom.